The present invention relates to new crystalline borosilicate compositions. Further, this invention relates to methods for producing these new crystalline borosilicate compositions and to a method for activating them to enhance their usefulness for certain catalytic conversion processes.
Zeolitic materials, both natural and synthetic, are known to have catalytic capability for various types of reactions, especially hydrocarbon conversions. The well-known crystalline aluminosilicate zeolites are commonly referred to as "molecular sieves" and are characterized by their highly ordered crystalline structure and uniformly dimensioned pores, and are distinguishable from each other on the basis of composition, crystal structure, adsorption properties and the like. The term "molecular sieves" is derived from the ability of the zeolite materials to selectively adsorb molecules on the basis of their size and form.
The processes for producing such crystalline synthetic zeolites are well known in the art. A family of crystalline aluminosilicate zeolites, designated ZSM-5, is disclosed in U.S. Pat. No. 3,702,886, said patent being herein incorporated by reference.
U.S. Pat. No. 3,941,871 relates to novel crystalline metal organosilicates which are essentially free of Group IIIA metals, i.e., aluminum and/or gallium. This patent is herein incorporated by by reference. It is noted therein that the amount of alumina present in the known zeolites appears directly related to the acidity characteristics of the resultant product and that a low alumina content has been recognized as being advantageous in attaining a low degree of acidity which in many catalytic reactions is translated into low coke making properties and low aging rates. A typical procedure for making the organosilicates is to react a mixture containing a tetraaklylammonium compound, sodium hydroxide, an oxide of a metal other than a metal of Group IIIA, an oxide of silicon, and water until crystals of said metal organosilicates are formed. It is also noted in the patent that the family of crystalline metal organosilicates have a definite X-ray diffraction pattern which is similar to that for the ZSM-5 zeolites. Minor amounts of alumina are contemplated in the patent and are attributable primarily to the presence of aluminum impurities in the reactants and/or equipment employed.
U.S. Pat. No. 3,884,835 discloses crystalline silica compositions. The crystalline silica materials may also contain a metal promoter which may be selected from Group IIIA, Group V B or Group VI B elements. Boron is disclosed as one of the metal promoters.
U.S. Pat. No. 4,088,605 is directed to the synthesis of a zeolite, such as ZSM-5, which contains an outer shell free from aluminum. The patent states at column 10, the paragraph beginning at line 20, that to produce the outer aluminum-free shell it is also essential that the reactive aluminum be removed from the reaction mixture. It is therefore necessary, as noted therein, to process the zeolite and to replace the crystallization medium with an aluminum-free mixture to obtain crystalization of SiO.sub.2 on the surface of the zeolite which can be accomplished by a total replacement of the reaction mixture or by complexing from the original reaction mixture any remaining aluminum ion with reagents such as gluconic acid or ethylene diamine tetraacetic acid (EDTA).
Crystalline borosilicate compositions are disclosed in U.S. patent application Ser. Nos. 733,267 and 836,403. These applications relate specifically to borosilicates and are prepared using the usual procedures for making the aluminosilicate zeolites. It is noted therein on page 12 of Ser. No. 836,403 that in instances where a deliberate effort is made to eliminate aluminum from the borosilicate crystal structure because of its adverse influence on particular conversion processes, the molar ratios of SiO.sub.2 /Al.sub.2 O.sub.3 can easily exceed 2000-3000 and that this ratio is generally only limited by the availability of aluminum-free raw materials.
While the art has provided zeolite catalysts having a wide variety of catalytic and adsorbtive properties, the need still exists for crystalline materials having different and/or enhanced catalytic properties. For example, an important use for a crystalline material is in conversion processes of oxygenated compounds such as the conversion of dimethyl ether to aliphatic compounds with a minimum amount of aromatics being formed. Additionally, many hydrocarbon conversion processes are performed employing zeolites, i.e., alkylation and isomerization. As is well-known in the art, it is important to maximize selectivity to the desired product and, as will be shown hereinbelow, the reaction of oxygenated compounds, e.g., dimethyl ether, and hydrocarbons, using compositions prepared by the method of the invention unexpectedly produce high selectivity to aliphatics and high activity which is contrary to that expected from a crystalline zeolite type composition containing a low level of alumina.